This invention relates to a method for displaying a radiographic image composed of a plurality of digital values, and more particularly to a method for generating a look up table (LUT) representing a desired gray scale transfer (GST) function for displaying a visually enhanced radiogram.
There exists significant activity in the development of digital x-ray image data capture systems. In such systems direct conversion to an electrical signal of the incident radiation is obtained using a plurality of sensors (also known as pixels) in an array. The sensor output is almost invariably immediately converted to a digital signal by an analog-to-digital converter as known in the art and further processed and stored in a databank for use in the eventual display of the data as a radiograph. U.S. Pat. No. 5,313,066 issued to Lee et al. (hereinafter the ""063 patent) and U.S. Pat. No. 5,315,101 issued to Hughes et al. describe typical such sensor arrays and their contents are incorporated herein by reference. Even though several different technologies are being utilized, the output data are quite similar.
A major advantage of digital data detection systems is the wide dynamic range of signal capture. Display media, such as radiographic film or cathode ray tube (CRT) displays, on the other hand, have a substantially more limited dynamic range. A typical digital x-ray capture system can have a useful dynamic detection range of greater than a 1,000:1. However, the typical currently available display media are generally limited to a dynamic range of less than 100:1. There is, therefore, need to determine and select the optimal limited range of useful data for diagnostic display, and then properly display such range on the available display medium.
This problem, which reduces to a need for a method whereby the exposure sensor output is mapped onto the display transfer function of the display device, has been addressed by the art in numerous ways. Typically the sensor output is digitized, and a histogram of the frequency of occurrence of digital values representing detected exposure is constructed. Histogram analysis is often used to determine the relevant portions of the data, that is data carrying significant diagnostic information, and the digital values in this range are mapped onto the display transfer function, usually using a look-up table (LUT), as is well known in the art. These steps are rather fundamental and are well known to the person skilled in this art. What is significant and the subject of continuing research is the manner in which the digital values are processed and mapped to display the maximum amount of relevant information within the limitations of the available display media. This in turn requires the generation of the transfer function to create an optimal radiograph wherein the features of interest are distinguishable from background features and wherein maximum visual contrast is applied to the region of interest.
The method most often used in actual practice for mapping the data derived from the detector, is to store a plurality of GST curves representing empirically derived GST shapes in the form of LUTs in a memory , and to select one of the stored GSTs to reproduce a given image. If the desired region of interest in the reproduction proves too dark, or has insufficient contrast, another GST curve is chosen and the output observed again and so on, until an acceptable image is produced. Such method, however, is limited by the number of curves stored, and does not allow for continuous changing of the contrast and brightness of the displayed image.
U.S. Pat. No. 4,641,267 issued Feb. 3, 1987 to Asai et al. shows an early attempt to overcome some of the above limitations. Asai begins by generating first a set of reference tone correction curves. In displaying an image, Asai selects one of the reference curves and a point Y(x0) on the curve corresponding to a desired point on the Y (output value) axis. By rotating the curve around this point Asai can change the contrast of the output image.
U.S. Pat. No. 5,946,407 issued Aug. 31, 1999 to Bamberger et al. is an improvement on the method taught by Asai and is believed to represent the current state in developing GSTs. Bamberger teaches creating the GST as a combination of two curves, by using two algorithms to describe two distinct portions of a GST. The point where the two curves connect is set at the 50% of the Y (output value) axis. The slope of the two curves around the connecting point determines the contrast of the displayed image in the area of most interest, while moving the connecting point along the X (input value) changes the brightness (or film density if the display medium is film) of the areas of interest. The term brightness as used hereinafter is to be considered as referring to the gray scale gradations of an image in general, and to include optical density when the display medium is hard copy.
While this method provides good results, it is computationally intensive. Furthermore, in certain applications it is desirable to calculate the point of maximum slope of the GST (maximum contrast) which is usually done by obtaining the second derivative of the curve. The discontinuity due to the use of two algorithms to form the GST introduces complications in this calculation. It is desirable, therefore to have a method for quickly developing accurate GSTs on demand, given certain input parameters representing desirable contrast and brightness characteristics with optimum visual appearance similar to the visual appearance obtained as a result of the typical HandD response of photographic materials used in capturing radiograms using the traditional method of exposing the photographic material to X-ray radiation.
It is an object of this invention to provide a method for generating on demand a gray scale transfer curve for use in displaying an image comprised of a plurality of digital values representing pixel gray scale levels with a desired contrast and brightness. This is achieved by using a single sigmoid shaped function Y=f(X, a, b, c) wherein the function resembles an HandD curve of a photographic film. The Y axis represents digital output values to be used in displaying an image, while the X axis represents input digital values representing captured image information.
xe2x80x9caxe2x80x9d is a number representing boundary conditions for X and Y, xe2x80x9cbxe2x80x9d is a first parameter controlling the location of the curve along the X axis and xe2x80x9ccxe2x80x9d is a second parameter controlling the slope of the curve slope. This function is used to derive a Look-Up-Table (LUT) representing a continuous GST curve having the desired contrast and brightness.
The above described method permits the derivation of families of GST curves and corresponding LUTs representing different contrasts and brightness, by varying the values of the parameters xe2x80x9cbxe2x80x9d and xe2x80x9ccxe2x80x9d.
Particularly useful is the function Y=a/(1+b*exe2x88x92cX). Parameter xe2x80x9ccxe2x80x9d is dependent on the maximum input digital value possible. Experience has shown that in a system where the input values have been normalized and range between 0 and 100, good results are obtained with values for xe2x80x9ccxe2x80x9d selected between 0.02% and 0.2% of the maximum scale value for X, and preferably between 0.06% and 0.10%. Parameter xe2x80x9cbxe2x80x9d is given by:
Xmax contrast*c=1n b
and the value of xe2x80x9caxe2x80x9d may be calculated by solving the above given function for xe2x80x9caxe2x80x9d using:
a=Yu*(1+b*exe2x88x92cXu).
The input digital values for the X and Y axis may be normalized preferably between 0-100, in which case xe2x80x9caxe2x80x9d is derived for Yu=100 and Xu=100.